The invention relates to a process and to an apparatus for the acquisition of operational data levels heralding the imminent approach to a given pre-determined upper lean-running limit during the operation of an internal combustion engine, specifically for the regulation of the internal combustion engine within the region of excess air, (.lambda.&gt;1).
At the present time, increased efforts are directed to permit internal combustion engines to function within preferably that operating range where the quantity of the harmful components of the exhaust gases can be kept small, and/or where the fuel consumption can be kept low, in order to meet the ever more stringent environmental rules regarding engine exhaust gases, and to respond to the challenge of the general shortage of fuel.
These requirements may be met by supplying the internal combustion engine with a comparatively lean fuel-air mixture, i.e., to tend toward combustion engine settings in the direction of a lean mixture, since operation in that region assures exhaust gases which are relatively free of harmful substances, and also assures low rates of fuel consumption. The precise determination of the operational point constituting the upper limit of the lean-running region is, therefore, of substantial significance, in order that the internal combustion engine may be operated under the constraint of the maximum admissible value, which differs for different engine rpm. As a result, it is of substantial significance that the operational point constituting the limit of the lean-running region be very accurately determined, and this determination can be based, for example, upon the fluctuations in the cyclic pressure patterns in the individual cylinders of the internal combustion engine. It is known that the dynamic stability (smoothness) of an internal combustion engine suffers, and become proportionately more disturbed, as one departs from an approximately stoichiometric relationship (.lambda. = 1). In the present case, only the departure which is in the direction of the region of excess air (.lambda.&gt;1) has any substantial significance. The numerical air-quantity .lambda. thus exerts a powerful influence upon the operation and the exhaust gas composition of Otto-engines in particular, and of internal combustion engines in general. One device which has become commonly known, and which may be called a .lambda.-probe, succeeds in monitoring the delivered fuel-air mixture whenever .lambda. = 1, through the analysis of the composition of the exhaust gases of the engine, but due to the technical characteristics of the .lambda. - probe, this monitoring process is admittedly feasible only at this one single value (.lambda. = 1). The conditions relevant to the emission of harmful exhaust-gas components (CO, HC, NOx), become substantially more favorable, as was already indicated earlier, when one manages to continue to operate the engine in the lean region, i.e., in a region which cannot, however, be surveyed with any precision by the .lambda.-probe. Such a continued lean operation of the combustion engine also results in the saving of fuel. It is rather difficult, however, to determine and measure the momentary .lambda. -values when .lambda. is greater than 1, that is, when the combustion engine is operated in the region of air excess. However, if the instantaneous air flow rate is known, a given, desired .lambda.-value can be obtained by a corresponding measured allocation of the fuel. Yet this scheme necessitates additional and continuous measurements, and, besides, the combustion process plays a decisive role in the use of the fuel, and, consequently, in the generation of the driving torques and the products of combustion. The present invention is based on the recognition that it should be feasible to monitor the combustion process and, thus, to measure the approach to a given desired value of .lambda. in the region of air excess, at least indirectly, since the required information is contained in a dynamic variable which is a function of the combustion process. In a particular instance, the combustion process is monitored via the variation occurring in the speed changes .DELTA..omega. of the crankshaft, i.e., through the measurement of the variation of the rotational speed of the crankshaft.
When one approaches the operational limits within the region of air excess, strong fluctuations are produced in the ignition delay and in the combustion behavior, such that the charge cannot be completely combusted in some of the individual power cycles. Moreover, combustion may fail to occur altogether, if ignition was not supplied prior to top dead center. If one succeeds in the early recognition of such retarded or incomplete combustions, then one can derive therefrom the data needed for the precise regulation and control of the internal combustion engine at a still-admissible .lambda. - values within the region of air-excess.
To clarify these matters, FIG. 1 depicts the curve I of the normal compression pressure of a combustion engine, having four cylinders in the example of the present embodiment, plotted over an axis corresponding to the respective angular position of the crankshaft. It may be seen that the pressure rises at or near zero degrees, i.e., as the piston approaches its top dead center, and that work must be expended to continue the motion past top dead center. The same thing happens at a crankshaft angle of 180.degree., when another piston of the four-cylinder engine reaches its top dead center. A graph of this kind may be obtained from a four-cylinder combustion engine by shutting off the ignition, or by interrupting the fuel delivery, and by cranking the engine with the starter.
During normal engine operation, that is to say, when both the fuel-air mixture and the ignition are supplied, a further pressure-surge takes place in the corresponding cylinder after a given piston moves through its top dead center, as portrayed by curve II in FIG. 1. The curves in FIG. 1 are merely qualitative; the pressure surge is due to fuel combustion and exerts a turning moment (torque) on the crankshaft. Thereafter, the rpm drops due to energy losses. The crankshaft's rotational velocity .omega., represented by curve III in FIG. 1, is a function of the power strokes of the internal combustion engine. The curve III shows that the rotational velocity of the crankshaft is subject to cyclic fluctuations; the magnitude of the rotational velocity .omega. is lowest (Region T1) before and during a given piston's arrival and presence at the top dead center position, whereas it is highest in the region T2, and continually decreases thereafter until the sequentially next piston arrives at its top dead center, in another cylinder. Since a four-cycle, four-cylinder internal combustion engine yields two power-strokes for each single revolution of the crankshaft, FIG. 1 accurately reflects the corresponding periodic .omega.-fluctuations of the crankshaft. As noted, these periodic fluctuations are functions of the rotating masses, and of the cyclic power sequences of the individual cylinders, whereby, as may be easily deduced, the amplitude of these periodic .omega.-fluctuations decreases as the engine speed (rpm) increases, since the power strokes occur ever more frequently, thus leaving less and less time for any reduction in the rotational velocity of the crankshaft. It is to be noted here, however, that this particular decrease of the .omega.-fluctuations shown in FIG. 1 is not linear. The periodic .omega.-fluctuations are at a given engine rpm and also occur when .lambda. is approximately equal to 1. Thus, the rpm oscillates about a mean value.
When the operating conditions of the combustion engine approach the operational limit within the region of excess air, (lean-running limit), strong fluctuations are produced in the ignition delay and in the combustion behavior which, in turn, cause momentary fluctuations in the angular speed of the crankshaft. Thus, in addition to the aforementioned periodic .omega.-fluctuations, further irregular .omega.-fluctuations occur, resulting in a more complex influence on the dynamic behavior of the rotational velocity of the crankshaft. Solely for those operational conditions where .lambda. is approximately equal to 1, and where the combustion progresses essentially uniformly and without delay, do the periodic .omega.-fluctuations predominate; the farther one enters into the region of excess air, the more do the irregular fluctuations outweigh the periodic fluctuations, i.e., the more erratic is the running of the internal combustion engine. The invention permits derivation of a signal which is representative of this erratic running of the engine, and this signal can be employed in the regulation and control of a particular operating point of the combustion engine.
During the approach to the lean-running limit, as the combustion becomes retarded, the value .DELTA..omega., or the differential-value between .omega..sub.min and .omega..sub.max, becomes conspicuously smaller, and the oscillations of the rotational velocity, or rpm, i.e., of .omega. about the mean value diminish accordingly and the invention exploits this phenomenon.